[Qemu-devel] Re: [PATCH] virtio: Use ioeventfd for virtqueue notify

[rukhsana ansari]

Hi,

W.r.t:
> Note that this is a tradeoff.  If an idle core is available and the
> scheduler places the iothread on that core, then the heavyweight exit is
> replaced by a lightweight exit + IPI.  If the iothread is co-located with
> the vcpu, then we'll take a heavyweight exit in any case.
>
Q: Does the kvm kernel code check for such a condition and take a
heavyweight exit?

> The first case is very likely if the host cpu is undercommitted and there is
> heavy I/O activity.  This is a typical subsystem benchmark scenario (as
> opposed to a system benchmark like specvirt).  My feeling is that total
> system throughput will be decreased unless the scheduler is clever enough to
> place the iothread and vcpu on the same host cpu when the system is
> overcommitted.
>
>
Q: Sorry if the answer is obvious here.
If the heavyweight exit is taken when both threads are assigned to the
same core, how will the system throughput increase?


Thanks
Rukhsana